Photographic printing



Filed June 17, 1957 INVENTOR DWIN R. CRAIG BY Az 512/ n ATTORNEY United States Patent 2,988,978 PHOTOGRAPHIC PRINTING Dwm R. Craig, Falls Church, Va., assignor to lJogetr-onics, Inc., Alexandria, Va., a corporation of Delaware Filed June 17, 1957, Ser. No. 666,122 6 Claims. (Cl. 95--73) This invention relates to photographic printing and particularly printing of the quality which ordinarily requires .dodging or masking.

Those familiar with photographic reproduction are well aware that one of the basic problems involved in satisfactory tone reproduction is caused by the relatively short exposure scale of printing emulsions. In addition to this problem, the laboratory technician must always cope with a photographic emulsion whose response characteristics are non-linear when resulting density is plotted against incident exposure. This non-linearity is characterized by the typical S-shaped curve usually found in plots of H and D emulsion characteristics. Because of these two inherent shortcomings, all high quality photographic reproduction is achieved only by the use of some form of dodging or masking.

The most satisfactory mask usually assumes the form of a photographic contact print on film made from the transparency to be reproduced. These masks fall in three categories, depending on their purpose and method of production. Most commonly used is the unsharp mask, which in effect is an out-of-focus positive transparency made from the original negative transparency. In practice, it is placed in contact with the transparency during final exposure and performs the function of reducing the apparent contrast in the original. The mask is usually made of material having a long exposure scale and is developed to a contrast somewhat less than that of the original. As a result, when the original and mask are superimposed, the combination still resembles the tonal structure and relationship that existed in the original, but with all densities uniformly compressed toward some intermediate value.

A second type of mask is called an area mask and, in all respects except its degree of unsharpness is the same as the unsharp mask. In the unsharp mask, a point image in the original might be reproduced as a circle of approximately one-sixteenth of an inch in diameter, whereas in the area mask, the same point might be reproduced as a disc anywhere from one-eighth to one-half inch in diameter.

A third type of mask, known as the highlight mask, is usually made on a material having a long toe characteristic followed by an extremely steep straight line portion, such as the films used in photomechanical reproduction of halftone screens. This mask is primarily used to produce contrast compression only in the thin areas of the originahand thus compensate for the non-linear characteristics of photographic emulsions to be used in subsequent steps of the process.

In the case of each of these marks, it is readily seen that their function is to cause the inverse image of the transparency to be superimposed on the transparency during printing. According to the present invention, instead of using a photographic mask in front of a uniform light source to accomplish this purpose, a light source is used whose brightness variations throughout its surface actually represent a positive luminous image of the transparency to be reproduced. More specifically, this invention contemplates a printing light source whose brightness pattern is selectively quenched by radiations whose pattern is determined by the pattern of the transparency from which a print is to be made.

The method involves the use of a light source comprising a phosphor or other suitable fluorescent material which emits actinic light, to be used in reproducing the transparency, which fluorescent material possesses the additional property of having its brightness subject to local modification by some form of incident radiation. Many fluorescent materials conforming to these requirements are commercially available and well known. Fluorescence of such materials can be achieved by high energy particles such as electrons, by high energy radiation such as ultraviolet light, by any visible radiation higher in energy than the emitted radiation, or by the drift of electrons in solid materials such as in electroluminescence. The color of the radiation emitted by such fluorescent materials may range from deep red to ultraviolet, depending on the composition and structure of the fluorescent material. A homogeneous phosphor usually emits a relatively homogeneous color of light, occupying only a narrow band of the spectrum. However, almost any color of emitted radiation can be achieved simply by suitably mixing the phosphors.

Another important characteristic of the fluorescent material used as the light source is its time constant or persistence. This factor determines the time elapsing between initial excitation of the fluorescent material and arrival at final brightness (build up) and also the time consumed between removal of excitation and arrival at minimum light level (decay). In practically all phosphors the build up time is very rapid, whereas decay time may range from a few microseconds to several hours or even days. In this application, the rate of decay is extremely important since it is this variable which permits the light source to portray an inverse image of the transparency to be reproduced. It is this natural decay characteristic which can be modified by incident radiation to control the brightness of a fluorescent material undergoing excitation. The decay rate of most phosphors is shortened by the incidence and absorption of infrared radiation, which is termed quenching, most predominant in phosphors having a naturally long decay rate.

The precise wave length of the infrared radiation which would be most effective for quenching a particular fluorescent material is probably a function of the dimensions within the lattice structure of the crystalline phosphor, and would be that wave length which would produce resonant vibration within the molecule. In summary, the method of the present invention contemplates the use of a quenching phosphor or other suitable fluorescent material as the printing light source, projecting an image of a transparency, using such a light source, and producing an inverse image of the transparency on the surface of the light source so that the inverse luminous image, when projected back through the transparency onto a photosensitive surface, provides in one step, photographic effects of the type previously produced by masking.

Accordingly, it is among the objects of the present invention to provide a photographic printing method comprising directing infrared light from a source of substantially uniform intensity through a transparency in one direction on a fluorescent substantially uniform intensity source of actinic light to quench selectively the fluorescent source, and directing unquenched actinic light from the fluorescent source through the transparency in an opposite direction on a photosensitive surface to expose the same. The transparency may be a photographic negative and the infrared light may be directed through the photosensitive surface or directed towards the transparency by reflection.

The photographic printing apparatus contemplated comprises a substantially uniform intensity source of fluorescent light, a support for a photosenstive body to be printed, a source of infrared radiation of substantially uniform intensity, and a support for a transparency disposed in an intermediate position in an optical path including the fluorescent source and photosensitive body support on the one hand and in an optical path including the fluorescent and infrared sources on the other. The optical paths may include a beam splitter which may assume the form of a dichroic mirror having a coating substantially totally reflecting radiation from the infrared source and substantially totally transmitting light from the fluorescent source. The photosensitive body may lie in each of these optical paths and it may be interposed between the infrared source and fluorescent source.

A more complete undertstanding of the invention will follow from a description of the accompanying drawing wherein:

FIG. 1 schematically depicts one form of the invention;

FIG 2 schematically depicts a modification; and

FIG. 3 schematically depicts another modification.

In the embodiment of the invention shown in FIG. 1, the light source it comprises an envelope 12 containing a cathode 14 and having a phosphor composition 16 deposited on the inner surface of its enlarged end 18 capable of being excited to luminescence by the absorption of electrons emanating from a flood gun structure including the cathode and accelerated by the application of a high voltage between the cathode 14 and enlarged end 18. A transparency Ztl, such as a photographic negative is supported in the path of fluorescent light emanating from the source 16, beyond which there is provided a projection lens 22 for forming an image of the transparency on a photosensitive surface carried by a support 26. The apparatus thus far described is similar to an ordinary enlarger with a diffuse light source and would produce comparable results. The entirely novel efifect is achieved by the introduction of quenching radiation.

As shown in FIG. 1, radiation from an infrared source 28 of substantially uniform intensity is directed through a filter 30, collected by a condenser lens 32, reflected by a beam splitter 34 through the projection lens 22, and transmitted through the transparency 20 to the proximate surface of the fluorescent material 16. The beam splitter is preferably a dichroic mirror operating on the interference principle and containing a deposited layer 36 whose thickness is chosen to cause substantially total reflection of the desired wave length of infrared and substantially total transmission of the unquenched portion of the actinic wave length of light from the source 16 which will be used to expose the photosensitive surface 24. With this arrangement, infrared illumination of the fluorescent material 16 can be considered as emanating from the projection lens 22. Accordingly, an image of the transparency 20 will be projected onto the fluorescent material 16 by infrared radiation. Since infrared of the proper wave length tends to quench the phosphor or other selected fluorescent material, thereby reducing its luminous emission, it follows that thin regions in the transparency will produce corresponding regions of low luminosity at the phosphor, whereas denser areas in the transparency will transmit less infrared and will reduce the luminosity of the fluorescent material to a smaller degree. Thus, in effect, a projected infrared beam passes through a negative transparency and forms a positive luminous image on the phosphor. Local brightness within the image projected on the photosensitive surface would then be a composite of local phosphor brightness and local density of the transparency. Accordingly, if the transparency and the phosphor were in intimate contact and if the contrast of the positive luminous image exactly matched the contrast of the negative transparency, then the image projected on to the photosensitive surface would be of uniform brightness completely devoid of image detail. This situation would correspond to that which would be obtained by use of a sharp mask, de-

veloped to a gamma of 1.0, in absolute register with the transparency. In practice, this situation can never be achieved with a mask, but appears to be entirely feasible, though probably notdesirable, with the apparatus contemplated herein.

The beam splitter 34 may assume the form produced by Bausch & Lomb Optical Company, whose catalog entitled Interference Filters includes among many others, the following examples of products applicable to the present invention:

Product Number: Traitsmits- 4247--55 4500 Angstrom 'units. 42--4756 5000 Angstrom units. 33-79-43 4360 Angstrom units. 337948 4860 Angstrom 337940 4050 Angstrom units.

which values will vary over a range of plus or minus Angstrom units. 7

The apparatus shown in FIG. 2 depicts the activating light source 40 more generally and contemplates the infrared source 28 as directing its beam directly through its filter 30 and the transparency 20 onto the phosphor 16, eliminating the need for the beam splitter of FIG. 1. The activating source 40 may produce ultraviolet light primarily or it may assume the form of a more conventional lamp producing some actinic light. The infrared source schematically shown may represent several sources whose combined effects are employed.

An application of this invention to a contact printer is exemplified in FIG. 3 where the transparency 20 and photosensitive material 24 carried by the support 26 are in intimate contact, with the actinic and infrared sources disposed on opposite sides thereof. In this case, infrared rays from the source 28 will pass through the filter 30, paper 26, emulsion 24, and transparency 20, to quench the fluorescent material 16 selectively to form apositive luminous image. This image in the form of actinic light is transmitted through the transparency to expose the photosensitive emulsion 24.

The apparatus of FIGS. 1, 2 and 3 will possess features of control corresponding to those customarily employed in masking, namely, control of percent masking and degree of unsharpness. Brightness of the infrared source may be varied to control the amount of quenching and thus to control contrast of the positive luminous image, corresponding to percent masking. The degree of unsharpness can be controlled by adjusting the space between the transparency and the fluorescent material, or by selection of the size of the aperture in the projection lens.

The advantages of the method and apparatus of the present invention over conventional masking include:

(1) Printing is achieved in one step requiring a minimum of time and materials.

(2) Problems of registering the mask and transparency are completely eliminated.

(3) Characteristics of the projected image can be observed directly and measured directly in the same apparatus in which the exposure occurs.

Whereas the method and apparatus thus described are intended primarily for black and white contact printing and enlarging, any applications to color work should not be excluded. Nor should the invention be restricted to the examples selected for purposes of description, beyond the scope of the appended claims.

I claim:

1. A photographic printing method comprising exciting a fluorescent surface to produce actinic light, directing infrared light uniformly through a transparency upon said surface to quench said surface selectively to form a luminous image, and simultaneously directing unquenched actinic light from said fluorescent surface through said transparency and forming an image of said surface on a photosensitive emulsion surface to expose the same.

2. A photographic printing method as set forth in claim 1 wherein said transparency is a photographic negative.

3. A photographic printing method as set forth in claim 1 wherein said infrared light is transmitted through said photosensitive emulsion surface.

4. A photographic printing method as set forth in claim 1 wherein said infrared light is directed towards said transparency by reflection.

5. A photographic printing method as set forth in claim 1 wherein the exciting and quenching intensity relationship is variable for controlling contrast.

6. A photographic printing method as set forth in claim 1 wherein the sharpness of said luminous image is varied with respect to said transparency.

References Cited in the file of this patent UNITED STATES PATENTS Christensen Dec. 15, 1925 Tuttle July 10, 1934 Urbach Sept. 27, 1949 Andreas et a1 Mar. 5, 1957 FOREIGN PATENTS Great Britain Oct. 5 ,1923 Great Britain July 11, 1939 Italy Feb. 20, 1951 Germany June 11, 1953 France Oct. 31, 1951 Notice of Adverse Decision in Interference In Interference No. 93,008 involving Patent No. 2,988,978, D. R. Craig, Photographic printing, final judgment adverse to the patentee was rendered Jan. 23, 1964, as to claims 1, 2 and 3.

[Ofiiaial Gazette April 28, 1964.]

Notice of Adverse Decision in Interference In Interference No. 93,008 involving Patent No. 2,988,978, D. R. Craig,

Photographic printing, final judgment adverse to the patentee was rendered Jan. 23, 1964, as to claims 1, 2 and. 3.

[Ofiicial Gazette April 28, 1.964.]- 

